1. Field of the Invention
The invention relates to antenna systems and more particularly to electronic scanning systems for scanning a given sector with a minimum number of radiating elements.
2. Description of the Prior Art
Beam scanning in electronically scanned antennas is achieved by controlling the excitation phase at the array elements to establish phase gradients across the array which determine the beam positions. In these systems the maximum scan angle that may be achieved without establishing grating lobes (additional principle lobes) in real space is determined by the interelement spacing in the array. A uniformly spaced array of isotropic elements may have a maximum scan angle of 90.degree. on either side of the perpendicular to the array surface when the spacing in the scanning plane is less than 1/2 wavelength. This scanning range is decreased, however, to a maximum of approximately 20.degree. on either side of the perpendicular when the spacing is increased to 3/4 of a wavelength. Because of this spacing limitation conventionally designed high gain electronically scanned antennas require a significant number of radiating elements with associated control and phase shift of components.
Early efforts for reducing the significant cost of electonic scanned arrays utilized small electronically steerable arrays located in the focal region of a microwave optical system. These systems, however, exhibited low aperature efficiency, because only a portion of the aperture was illuminated for each scan angle.
Significant improvements in aperature efficiency and antenna component reductions were realized with the development of the overlapping subarray technique. This technique uses appropriate combinations of orthogonal beamformers and switching networks to achieve the desired scanning capability and beam characteristics. In these designs the primary collimating device is a lens or reflector with subarraying networks, such as, Butler matrices or Rotman lenses having apertures located in the focal regions. These antennas exhibit the unfavorable characteristics of a physically deep configuration which is concomitant with optically fed array systems. This physical depth may be reduced by substituting a Butler matrix for the primary collimating lense. This is not an attractive approach for large aperature antennas because of the complexity of the Butler matrix.
Another approach uses partially overlapped or interlaced subarrays. These, however, exhibit poor side lobe performance with reduced scanning capabilities relative to the fully overlapped subarrays.
Many of the shortcomings of the above prior art system are overcome by the invention disclosed in U.S. Pat. No. 4,507,662 assigned to the assignee of the present invention. In this device radiating elements of an antenna array are correspondingly coupled to a second array having element spacing substantially smaller than that between the radiating elements. This second array is space coupled, through a space coupling region, to a third array, which is scannable in the space coupling region. The third array has fewer elements than the second array and is approximately of the same physical size and length. Each scan angle of operation of the third array establishes a phase distribution across the second array, which is coupled to radiating array, thereby providing radiation in free space, at a scan angle corresponding to the scan angle of the third array. Since the feed has fewer elements than the radiating array, an element and associated component savings are realized. The element saving, however, is somewhat offset by the additional elements utilized in the space coupling region.